1. Field of the Invention
This invention relates to a control apparatus, a control method, a control unit, and an engine control unit, for controlling a controlled object by employing a ΔΣ modulation algorithm or ΣΔ modulation algorithm.
2. Description of the Prior Art
Conventionally, a control apparatus of the aforementioned kind has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-154704. The control apparatus includes detection means for detecting an output from a controlled object (i.e. controlled system) and then outputting the result of detection as a detection signal of an analog quantity, reference input-calculating means for calculating a reference input as a deviation of the detection signal from a target value of the analog quantity input from a host system, conversion means for converting the calculated reference input to a 1-bit digital signal using a ΔΣ modulation algorithm, and compensation means for performing compensation for the 1-bit digital signal delivered from the conversion means and inputting the resulting signal as a control input to the controlled object (see FIG. 6 of the aforementioned publication).
In this control apparatus, the reference input (analog quantity) as the deviation of the detection signal from the target value is calculated by the reference input-calculating means, and converted to the 1-bit digital signal by the conversion means, and then the 1-bit digital signal is compensated for by the compensation means, followed by being input into the controlled object as the control input. In the above process, the control input having an opposite phase to the reference input is generated so as to cancel out the deviation of the output from the controlled object, from the target value, and input into the controlled object. The output from the controlled object is thus feedback-controlled such that it converges to the target value.
Further, in the ΔΣ modulation algorithm, the integral of an input deviation, i.e. a deviation of a modulation output from the ΔΣ modulation algorithm from the reference input to the ΔΣ modulation algorithm is calculated and quantized, and the quantized value of the input deviation is output as the modulation output. More specifically, the modulation output is calculated as a value of 1 when the integral of the input deviation is equal to or larger than 0, and as a value of −1 when the same is smaller than 0.
According to the above control apparatus, depending on the characteristics of the controlled object, the absolute value of the reference input sometimes becomes larger than the absolute value of the modulation output. In such a case, the absolute value of the integral of the input deviation is excessively increased. If the absolute value of the integral of the input deviation is excessively increased, even if the sign of the input deviation is inverted due to the inversion of the sign (plus or minus) of the reference input, the sign of the modulation output is maintained without inversion until the absolute value of the increased integral is sufficiently reduced. In other words, there occurs dead time between inversion of the sign of the reference input and inversion of the sign of modulation output, which decreases the convergence of the output from the controlled object to the target value, leading to the degraded controllability. Further, in a case where the ΣΔ modulation algorithm is used in place of the ΔΣ modulation algorithm, when the absolute value of the reference input becomes larger than the absolute value of the modulation output, the absolute value of the integral of the reference input becomes larger than the absolute value of the integral of the modulation output, which causes the difference or deviation between the integral of the reference input and the integral of the modulation output to be excessively increased. Therefore, even if the sign of the reference input is inverted, the sign of the deviation, i.e. the sign of the modulation output is maintained without inversion until the absolute value of the integral of the reference input becomes smaller than the absolute value of the integral of the modulation output. This raises the same problem as described above.